Inhibition of hematite on acid mine drainage caused by chalcopyrite biodissolution

Baojun Yang,Wen Luo,Maoxin Hong,Jun Wang,,Xueduan Liu,Min Gan,Guanzhou Qiu

1 School of Minerals Processing and Bioengineering,Central South University,Changsha 410083,China

2 Key Laboratory of Biohydrometallurgy,Ministry of Education,Central South University,Changsha 410083,China

3 Department of Dermatology,The First Hospital of Changsha,Changsha 410005,China

Keywords:Chalcopyrite Hematite Biodissolution Acid mine drainage Acidithiobacillus ferrooxidans

ABSTRACT Even though biodissolution of chalcopyrite is considered to be one of the key contributors in the formation of acid mine drainage(AMD),there are few studies to control AMD by inhibiting chalcopyrite biodissolution.Therefore,a novel method of using hematite to inhibit chalcopyrite biodissolution was proposed and verified.The results indicated that chalcopyrite biodissolution could be significantly inhibited by hematite,which consequently decreased the formation of AMD.In the presence of hematite,the final biodissolution rate of chalcopyrite decreased from 57.9% to 44.4% at 20 day.This in turn suggested that the formation of AMD was effectively suppressed under such condition.According to the biodissolution results,mineral composition and morphology analyses,and electrochemical analysis,it was shown that hematite promoted the formation and accumulation of passivation substances (jarosite and Cu2-xS) on chalcopyrite surface,thus inhibiting the biodissolution of chalcopyrite and limiting the formation of AMD.

1.Introduction

Mining industry has made significant contribution to the development of global economy,but also caused serious environmental problems of acid mine drainage(AMD)[1].It is believed that a single mine can produce hundreds to thousands of cubic meters of AMD [2],and the generation of AMD can persist for hundreds to thousands of years [3].Acid mine drainage,characterized by high concentrations of heavy metals and sulfate,will pose a major threat to the environment and human health [4,5].Therefore,it is extremely urgent to control and treat the AMD caused by mine operation.Pyrite,as the most abundant sulfide mineral on the earth,is considered to be the main cause of AMD [6].Therefore,researches on controlling AMD have mainly focused on the inhibition of pyrite dissolution,while ignoring the inhibition of other sulfide minerals dissolution.For example,the wastes generated by gold and porphyry copper mining and mineral processing operations(e.g.,waste rocks,tailings,leaching residue,sludges,etc.)typically contain chalcopyrite and arsenopyrite [7,8],which are important minerals that produce acid and release arsenic and copper in some deposits,and they are also important sources of AMD[9–11].Excessive concentrations of arsenic and copper will threaten the ecological environment and human health [12,13].

Being the most abundant copper sulfide mineral and main copper resource[14,15],chalcopyrite has attracted extensive attention in copper extraction.So far,the main process for extracting copper from chalcopyrite is pyrometallurgy,although hydrometallurgy is sometimes used[16].Compared with pyrometallurgy and conventional hydrometallurgy,chalcopyrite bioleaching has incomparable advantages of simple operation,mild reaction,friendly environment and low energy consumption,which has attracted extensive attention of scholars [17].However,chalcopyrite has complex composition,high lattice energy and strong passivation effect,which is not conducive to bioleaching.To enhance chalcopyrite bioleaching,many studies have been devoted to revealing the bioleaching mechanism of chalcopyrite [18,19].Based on the bioleaching mechanism,various optimization methods,such as adjusting the optimal redox potential and pH value,using mixed thermophilic bacteria,and adding activated carbon,silver ion and surfactant,have been used to enhance chalcopyrite bioleaching[20].These methods have improved the bioleaching efficiency of chalcopyrite and promoted the large-scale application of chalcopyrite bioleaching.Up to now,nearly a quarter of the world’s copper production is obtained through bioleaching[21].The application of chalcopyrite bioleaching to extract copper can effectively alleviate the shortage of copper resources.However,large quantity of lowgrade chalcopyrite waste exposed during the mining process can be dissolved by bacteria or chemical oxidation,releasing tremendous amount of copper ions,iron ions and acids both of which are important chemical components of AMD[10,11].It is estimated that more than 30 million tons of copper tailings have been produced all over the world [22].Therefore,some researchers believe that chalcopyrite is also an important reason for the formation of AMD [10,11,23].Acidophilic microorganisms can promote the regeneration of ferric ions,copper ions,and ferrous ions.Ferric ion is an important oxidant for the dissolution of chalcopyrite,and ferrous-copper couple has the synergistic effect in the oxidation of chalcopyrite [24].Therefore,the oxidative dissolution rate of chalcopyrite participated by acidophilic microorganisms can be six orders of magnitude higher than the equivalent chemical rate [25].Hence,biodissolution of chalcopyrite accelerates the release of copper ions,iron ions,and acids,as well as the formation of AMD,which requires urgent resolution by inhibiting such process so as to improve the environment of the mining sites effectively.

Fig.1.Laser particle size distribution of the mineral samples:(a) chalcopyrite sample,(b) hematite sample.

Since the oxidation process of sulfide mineral requires the participation of water,oxygen and microorganisms (e.g.,iron and/or sulfur oxidizing bacteria),removing any of these three factors can inhibit the oxidation and dissolution of sulfide mineral[26,27].Thus,by leverage on this hypothesis,various methods have been developed to suppress the oxidation of sulfide minerals to mitigate the pollution of AMD.One of the strategies involves the passivation of sulfide mineral,that is,forming chemical protective coating on the surface of sulfide mineral to prevent it from contacting with oxidants such as water,air,bacteria and iron ions.To date,various passivators are used to inhibit the oxidation of sulfide mineral,including silicate[28],phospholipid[29],phosphate[30],and triethylenetetramine [31].Although these passivators are generally effective,they also have their own disadvantages.For instance,the formation of phosphate,phospholipid,silicate passivation coating requires the prior oxidation of sulfide mineral surface by hydrogen peroxide [32],limiting the large-scale application of these techniques.Triethylenetetramine has biological toxicity,which makes it unsuitable for field application.In recent years,some scholars have proposed the use of microcapsules formed by metal organic complexes to inhibit the oxidation of sulfide minerals [9,33–35].These methods have achieved good results,effectively inhibiting the oxidation of sulfide minerals.However,these methods require the use of catechol,which is cytotoxic and may cause secondary pollution to the environment.

In order to overcome these shortcomings,it is necessary to explore alternative methods that use fewer chemical reagents and are more environmentally friendly.Iron oxyhydroxides/oxides such as hematite are ubiquitous in nature,especially in mining sites[36,37].Previous studies have shown that hematite can affect the electrochemical properties of sulfide mineral through the formation of surface bound species,thereby inhibiting the chemical oxidation of sulfide mineral [36,38].Due to its natural origin,easy availability,and environmental friendliness,hematite seems to be a good choice for inhibiting the oxidation of chalcopyrite.However,some studies have shown that hematite can mediate the transfer of electrons to microorganisms[39,40],which may accelerate the oxidation of chalcopyrite by microorganisms.Therefore,it is not clear whether hematite can inhibit the biooxidation of chalcopyrite,which requires more in-depth research and investigation.

Fig.2.XRD spectra of the mineral samples:(a) chalcopyrite sample,(b) hematite sample.

Based on the previous researches,the objectives of this work were to:(1)study the use of hematite to control AMD by inhibiting chalcopyrite biodissolution;and(2)elucidate the inhibition mechanism of hematite on chalcopyrite biodissolution.In order to better understand the role of hematite in the biodissolution of chalcopyrite,X-ray diffraction (XRD),scanning electron microscopy (SEM),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy (FT-IR),and cyclic voltammetry analysis (CV) were used to investigate the surface changes of chalcopyrite.

2.Materials and Methods

2.1.Biodissolution experiments

The high purity chalcopyrite and hematite obtained from Tonglushan,Daye City,Hubei Province,China were used in this research.Mesophilic bacterium (A.ferrooxidansATCC 23270)obtained from the Key Lab of Biohydrometallurgy,Ministry of Education,Central South University was used in the biodissolution experiments.The culture medium and culture conditions of the strain were the same as described in our previous study [41].250 ml flasks containing 100 ml sterile 0 K medium and 2 g chalcopyrite powder were used for biodissolution experiments,and the experiments were conducted for 20 days.An initial pH of 2.0 was set for the experimental system,which was approximately similar to the pH of the wastewater in the mining sites [42,43].Flasks inoculated with 4.0×107cells/ml bacteria were cultured in a shaker at 170 r?min-1and 30°C.The effect of hematite on the biodissolution of chalcopyrite was examined by employing various hematite concentrations (0 g?L-1and 1 g?L-1).Similar conditions to the biodissolution experiment were used in the sterile control experiments.Each experiment was carried out three times under similar conditions to ensure the reliability of the results.A pH meter (PHS-3C) was used to measure the pH of the solution in the dissolution system every four days.To monitor the metal ions concentration and bacteria concentration of the dissolution system,200 μl samples were extracted every four days.Deionized water was added to the dissolution system to offset the evaporation loss,while 0 K medium with pH of 2.0 was added to the dissolution system to offset the sampling loss.

Fig.3.Variations of (a) dissolution rate of copper,(b) total iron concentration,(c) bacterial concentration,and (d) pH during chalcopyrite dissolution.

Fig.4.SEM images of the residues after biodissolution:(a),(b)residues of the biodissolution control system;(c),(d)residues of the biodissolution system containing 1.0 g?L-1 hematite.

2.2.Cyclic voltammetry experiments

Cyclic voltammetry experiments were conducted using a single-chamber three-electrode system,which consisted of platinum electrode as counter electrode,Ag/AgCl in saturated KCl as reference electrode,and glassy carbon disk(φ=3 mm)as working electrode.The electrolyte used in the electrochemical test was sterile 0 K medium (pH=2.0).The working electrode was fabricated by slurry coating technique.The slurry was prepared by mixing mineral powder (10 mg),5% Nafion solution (400 μl),and anhydrous ethanol (600 μl).After that,10 μl of the slurry mixture was coated on the polished glassy carbon electrode to construct the working electrode,and then the working electrode was blown dry with nitrogen for subsequent cyclic voltammetry tests.Before cyclic voltammetry tests,ultrapure nitrogen was pumped into the electrolyte for 20 min to discharge dissolved oxygen.Cyclic voltammetry tests were carried out at a scanning rate of 20 mV?s-1.The scanning route started from open circuit potential (OCP),progressed to 800 mV,continued to -600 mV,and finally returned to OCP.

Fig.5.XRD spectra of the biodissolved residues:(a) residues treated without the presence of hematite,(b) residues treated in the presence of 1.0 g?L-1 hematite.

2.3.Analytical techniques

BCO spectrophotometry was employed to determine the copper ion concentration,whileo-phenanthroline spectrophotometry was employed to determine the iron ion concentration in the solution[44].Through the above method,the detection limit of copper ion and iron ion was 0.5 mg?L-1,and the error range was within 5%.Concentration of free cells in the solution was measured using hemocytometer under optical microscope (CX31,magnification 400 times).After the completion of the biodissolution experiment,qualitative filter paper was used to filter the biodissolution residues,and the filtered residues were washed with deionized water repeatedly.After which,they were placed in a DZF-6050 oven for future characterization analyses.XRD (X’Pert Pro,PNAalytical,Netherlands) and FT-IR (Nicolet Nexus670) were used to analyze the mineralogical composition of the biodissolution residues.The surface morphology of the biodissolution residues was observed by SEM (Quanta 650 FEG).XPS (ESCALAB 250Xi) was employed to analyze the Fe and S species on the surface of pristine ore/biodissolution residues.The methods of charge correction and split peak fitting of XPS are as described in our previous research [45].

3.Results and Discussion

3.1.Effect of hematite on the biodissolution of chalcopyrite

Ore samples were crushed and dry milled first,and then wet sieved with 200-mesh sieve.The results of laser particle size analysis showed that more than 98% of the sieved chalcopyrite and hematite particles were less than 70 μm (Fig.1).Then,the sieved mineral samples were vacuum dried and stored in a nitrogen environment,before subjecting them to subsequent dissolution experiments.According to Fig.2,XRD analysis showed that the phase composition of the chalcopyrite sample was mainly chalcopyrite(CuFeS2,PDF#71-0507) and trace quartz (SiO2,PDF#33-1161),while the phase composition of hematite sample was mainly hematite (Fe2O3,PDF#85-0987).The elemental composition of chalcopyrite samples analyzed by X-ray fluorescence spectroscopy(XRF)contained Cu(33.17%),Fe(28.12%),S(30.98%),O(5.42%),and other elements(2.31%),while the elemental composition of hematite sample contained iron (46.67%),oxygen (36.3%),silicon(9.11%),aluminum (5.43%),and other elements (2.49%).These results showed that the chalcopyrite and hematite samples used in this study have high purity.

The changes in the dissolution rate of copper,total iron concentration,free cell concentration,and pH during the dissolution of chalcopyrite were shown in Fig.3.It can be observed that the final dissolution rate of copper in the biodissolution system decreased from 57.9% to 44.4% in the presence of 1.0 g?L-1hematite (Fig.3(a)).As such,it is suggested that hematite could inhibit the biodissolution of chalcopyrite effectively,thereby alleviating AMD pollution caused by the release of copper ions and acid.For the sterile systems,the final dissolution rates of copper were onlyca.6.5%,which were significantly lower as compared to that in the biodissolution system.It is worth mentioning that in the middle and late stages of the biodissolution of chalcopyrite,the dissolution rate of copper in the biodissolution system containing hematite hardly increased,while the dissolution rate of copper in the biodissolution control system still maintained a rapid growth rate.This phenomenon indicated that hematite treatment intensified the passivation of chalcopyrite during the biodissolution process.

Fig.3(b) presented the trends of the change in the total iron concentration during the dissolution of chalcopyrite.In the early stage of the biodissolution of chalcopyrite,the total iron concentration of the biodissolution system increased rapidly due to the oxidative decomposition of minerals by bacteria,and then decreased gradually because of the formation of jarosite that consumed ferric ions(Eq.(5)).As a result of the dissolution of minerals by acid,the total iron concentrations in the sterile systems increased slowly,which were obviously lower as compared to that in the biodissolution systems.The total iron concentration of the biodissolution system containing hematite was significantly lower as compared to that in the biodissolution control system.This may be because hematite treatment facilitated the generation of jarosite,resulting in the significant consumption of ferric ions.Eqs.(1)-(5) showed the main biochemical reactions in the biodissolution of chalcopyrite [46].It has been widely recognized that dissolved iron ions are the most critical parameter that can influence the biodissolution of chalcopyrite [41,47].Therefore,a lower concentration of total iron is not conducive to the biodissolution of chalcopyrite.In this way,by facilitating the precipitation of iron ions,hematite can inhibit the biodissolution of chalcopyrite.

Fig.3(c) showed the variation of free cell concentration during the biodissolution of chalcopyrite.The free cell concentration of the system with hematite addition was lower than that of the control system.We speculated that this might be because hematite reduced the energy substance on chalcopyrite surface and in solution by promoting the passivation of chalcopyrite,thus inhibiting the growth of bacteria and further inhibiting the biodissolution of chalcopyrite.The variations of pH are shown in Fig.3(d).The pH value of sterile controls continued to increase above 2.32 due to acid consumption by mineral dissolution,as indicated by Eqs.(2) and (3).In the first four days,pH of the biodissolution system increased continuously as a result of acid consumption during mineral dissolution(Eqs.(2) and (3)).After which,it decreased gradually because of the large amount of acid generated during the bacterial oxidation of mineral and precipitation of jarosite (Eqs.(4)and(5)).It can be observed that the pH value of the biodissolution system containing hematite was higher as compared to that of the biodissolution control system,which indicated that hematite inhibited chalcopyrite biodissolution to release acid.Furthermore,it was indicated by some studies that when the pH of biodissolution system was less than 3.5,more jarosite would be formed with the increase of pH value [48,49].Thus,the formation of jarosite is facilitated at a higher pH value of the biodissolution system,which would inhibit the biodissolution of chalcopyrite.

Fig.6.Zeta potential of the mineral samples.

Fig.7.XPS spectra of the pristine chalcopyrite and its biodissolved residues:(a)Cu 2p3/2 spectra of pristine chalcopyrite,(b)Cu 2p3/2 spectrum of residue without hematite treatment,(c) Cu 2p3/2 spectrum of residue treated with 1 g?L-1 hematite;(d) Fe 2p3/2 spectrum of pristine chalcopyrite,(e) Fe 2p3/2 spectrum of residue treated without hematite,(f) Fe 2p3/2 spectrum of residue treated with 1 g?L-1 hematite.

According to the above results,hematite can encourage the generation of passivation layer (jarosite) on chalcopyrite surface and inhibit the growth of bacteria.In such way,biodissolution of chalcopyrite can be inhibited,and thereby reducing the generation of AMD.

3.2.Mineral morphology and phase analysis

Fig.8.XPS spectra of the pristine chalcopyrite and its biodissolved residues:(a)O 1s spectrum of pristine chalcopyrite,(b)O 1s spectrum of residue treated without hematite,(c) O 1s spectrum of residue treated with 1 g?L-1 hematite;(d) S 2p spectrum of pristine chalcopyrite,(e) S 2p spectrum of residue treated in absence of hematite,(f) S 2p spectrum of residue treated in presence of 1 g?L-1 hematite.

SEM was used to observe the surface morphologies of minerals after biodissolution (Fig.4).After biodissolution,the surface of chalcopyrite was covered by a large number of micro-particles and became rough.These micro-particles with a diameter of 100–1000 nm may be passivation substances covered on the surface of chalcopyrite.Compared with the control system,the surface of the residue treated with hematite was covered with more micro-particles.These results showed that chalcopyrite was passivated during the biodissolution process,and hematite treatment aggravated the passivation of chalcopyrite surface.Therefore,hematite can inhibit chalcopyrite biodissolution by promoting the formation of passivation substances.The XRD phase retrieval analyses of the biodissolution residues were shown in Fig.5.Chalcopyrite (CuFeS2,PDF#71-0507),jarosite (KFe3[SO4]2(OH)6,PDF#71-1777) and quartz (SiO2,PDF#33-1161) were identified as the main mineral phase compositions of the residues in the biodissolution control system(Fig.5(a)).For comparison,the main mineral components of the residues in the biodissolution system containing hematite were chalcopyrite (CuFeS2,PDF#71-0507),jarosite (KFe3[SO4]2(OH)6,PDF#71-1777),hematite (Fe2O3,PDF#85-0987) and quartz (SiO2,PDF#33-1161) (Fig.5(b)).These results indicated that hematite had not been dissolved during the chalcopyrite biodissolution process.This undissolved hematite might adhere to the surface of chalcopyrite and hinder the contact between chalcopyrite and oxidant,thereby inhibiting the dissolution of chalcopyrite.To verify this hypothesis,we tested the zeta potential of chalcopyrite and hematite under acidic conditions.The results showed that the zeta potentials of chalcopyrite and hematite were both positive when the pH was below 2.8 (Fig.6),indicating that there was almost no physical adsorption between chalcopyrite and hematite in this system.Elemental sulfur was barely detected in the biodissolution residues,which was due to the oxidization of sulfur to sulfuric acid byA.ferrooxidansduring the prolonged biodissolution process(Eq.(4))[50].Compared with the pristine chalcopyrite(Fig.1),jarosite was produced on the surface of the biodissolution residues,and the additional hematite further promoted the formation of jarosite.Previous research has shown that hematite could promote the formation of thick and mechanically strong oxide layer on pyrite that inhibited its oxidation even under acidic conditions[51].In this study,hematite has a similar effect,which can promote the formation of passivationlayer (jarosite),thereby inhibiting the biodissolution of chalcopyrite.This was consistent with the results obtained from the dissolution experiments and SEM analysis.

3.3.Surface elemental composition analysis

In order to explore the chemical state of mineral surface,original chalcopyrite and residues were analyzed by XPS.The XPS spectra of Cu 2p,Fe 2p,S 2p,and O 1s were presented in Figs.7 and 8,and the corresponding curve fitting parameters were summarized in Table 1.Fig.7(a)-(c) showed the Cu 2p3/2peaks on the mineral surface.In the original chalcopyrite (Fig.7(a)),the Cu 2p3/2peaks located at 932.1 eV and 932.9 eV corresponded to CuFeS2in the mineral lattice and Cu2S species,respectively [52].Cu2S species was attributed to secondary mineral formed by oxidation on the surface of chalcopyrite.In biodissolved residues (Fig.7(b) and(c)),the peaks of Cu2p3/2appearing at 931.9 and 934.0 eV corresponded to CuS and CuO species,respectively [52].After biodissolution,almost all chalcopyrite on the mineral surface was oxidized to CuS and CuO species,and more CuO species were formed on the mineral surface treated with hematite.

Table 1 XPS peak parameters for Cu 2p,Fe 2p,S 2p,and O 1s spectra

Fig.9.FT-IR spectra of original chalcopyrite and biodissolved residues.

Fig.10.CV profiles of chalcopyrite residues treated with or without hematite.

Fig.7(d)-(f)presented the Fe 2p3/2peak patterns of the pristine chalcopyrite and its residues after biodissolution.There were four types of Fe coordination and oxidation states in the fitted Fe 2p3/2spectra:(i) iron in the mineral phase (Fe(II)-CuS2,707.7 eV) [53],(ii)ferrous oxide species(Fe(II)-O,709.7 eV),(iii)iron oxide species(Fe(III)-O,(711.4 ± 0.2) eV)[54],and (iv) jarosite (Fe(III)-SO,(713.2 ± 0.2) eV) [51].Due to slight oxidation during the grinding process,there were some Fe(III)-O species on the surface of chalcopyrite in addition to Fe(II)-CuS2species (Fig.7(d)).After biodissolution,most of Fe(II)-CuS2species was transformed into Fe(III)-O and Fe(III)-SO species,that is,the surface of chalcopyrite was seriously oxidized (Fig.7(e) and (f)).The oxygen coordination and species on the surface of original chalcopyrite contained Fe—O(-II),Fe—OH and H2O,while only Fe—OH existed in biodissolution residues(Fig.8(a)-(c)).This result further showed that the surface of chalcopyrite was seriously oxidized during the process of biodissolution.Compared with the control group,the surface of the residue treated with hematite contained more Fe(II)-CuS2,Fe(II)-O and Fe(III)-SO(jarosite)species,indicating that hematite promoted the formation of jarosite and inhibited the oxidation of chalcopyrite.

Fig.11.Possible inhibition mechanism of hematite on chalcopyrite biodissolution.

The FTIR spectra of the pristine chalcopyrite and its residues after biodissolution were shown in Fig.9.The strong broadband corresponding to OH stretching of jarosite was observed at 3393 cm-1[59].Four peaks located at 1198,1085,1004,and 632 cm-1could be attributed to the S-O bending vibration in jarosite [60].For the pristine chalcopyrite,no characteristic peak of jarosite could be observed.This indicated that the pristine chalcopyrite exhibited high purity,without any existence of jarosite.After biodissolution of chalcopyrite,strong characteristic peaks of jarosite appeared on the residues surface,especially in the hematite treatment system.These results indicated that hematite promoted the generation of jarosite,thereby increasing the passivation of chalcopyrite.

3.4.Cyclic voltammetry analysis

Electrochemical methods are usually used to speculate the redox reactions that may occur during the mineral dissolution process.The electrochemical dissolution kinetics and thermodynamics of chalcopyrite under different conditions can be explored by CV.The intensity of the redox reaction on the surface of the working electrode is directly related to the current intensity of the CV.The CV profiles of chalcopyrite residues in 0 K medium were presented in Fig.10.These CV curves contained two cathodic peaks(C1 and C2) and three anodic peaks (A1,A2 and A3).Many researchers believed that the redox reactions on the surface of the working electrodes corresponded to the anodic and cathodic peaks in the CV profile [61,62].The oxidation of H2S to S can be assigned to the anodic peak A1 near 0 mV(Eq.(6))[63].The anodic peak A2 between 200 and 400 mV is associated with the oxidation of chalcocite to Cu2-xS (Eq.(7)) [64].The A3 peak around 650 mV represents the decomposition of chalcopyrite into intermediate species (Eq.(8)) [65].The reduction of ferric ions to ferrous ions can be assigned to the cathodic peak C1 near 350 mV (Eq.(9))[66].The cathodic peak C2 observed between -200 and 0 mV is related to the reduction of copper ions and sulfur to covellite and metallic copper,respectively (Eqs.(10)–(11)) [67].The current intensity of the anodic peak A2 (corresponding to the oxidation of chalcocite to Cu2-xS)of the residue electrode treated with hematite was significantly higher as compared to that of the residue electrode treated without hematite.While the current intensity of the other peaks of the residue electrode treated in the presence of hematite was lower as compared to that of the residue electrode treated in the absence of hematite.Some scholars believed that Cu2-xS was one of the passivation factors of chalcopyrite during the biodissolution [20,68].These results indicated that hematite treatment promoted the formation of non-stoichiometric copper sulfide,thereby hindering the redox reaction on the surface of chalcopyrite.

According to the above results,the mechanism of hematite inhibiting the biodissolution of chalcopyrite was illustrated in Fig.11.It was revealed that hematite promoted the formation and accumulation of passivation substances (jarosite and Cu2-xS)on the surface of chalcopyrite,thereby inhibiting the biodissolution of chalcopyrite and reducing the generation of AMD.

4.Conclusions

In this work,an effective novel strategy to inhibit the biodissolution of chalcopyrite was proposed via the use of hematite.The results indicated that hematite significantly inhibited the biodissolution of chalcopyrite.When hematite was present,the biodissolution rate of chalcopyrite was reduced from 57.9% to 44.4%.This in turn suggested that hematite could effectively suppress the generation of AMD under these conditions.Hematite promoted the formation and accumulation of passivation substances (jarosite,and Cu2-xS) on chalcopyrite surface,thus inhibiting the biodissolution of chalcopyrite and reducing the generation of AMD.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Natural Science Foundation of Hunan Province (No.2018JJ1041) and National Natural Science Foundation of China (Nos.51774332,U1932129,51804350 and 51934009).